1. Field of the Invention
The present invention relates to a disc reproducing apparatus which is useful for reproducing data stored on a disc. More specifically, the invention relates to a disc reproducing apparatus which is able to read out the data stored on the disc in a short time.
2. Description of the Related Art
In the field of sound apparatus, much interest exist in the sound apparatus for reproducing a digital audio signal stored on an optical disc wherein the digital audio signal is obtained by PCM(Pulse Code Modulation) which is a kind of A/D conversion.
A CD(Compact Disc) system is a kind of sound apparatus for reproducing or recording a digital audio signal on the optical disc. The diameter of the CD is 12 centimeters. Apparatus for reproducing the digital audio signal in the CD system consist of an optical pickup device to read out the data.
A CD system is essentially the sound apparatus for reproducing or recording a digital audio signal. But, the use of the CD system is not only for sound apparatus, but also as a data reproducing apparatus for reproducing computer data. The CD is a storage medium which has high storage density. Therefore, a CD-ROM system has been developed wherein the computer data is stored on the storage area of the CD.
The storage format of the CD-ROM system is the same as the Storage format of the CD system. Each disc stores a stereo audio signal comprised of two 16 bit data words sampled at 44.1 kHz in the CD system.
In the CD-ROM system, the minimum data block stored on the disc is an 8 bit (1 byte) data word which is obtained by dividing a 16 bit data word in two. A one sector data word is composed of 2352 byte data words.
The one sector data word is composed of:
(1) a 12 byte data word which is synchronous data for identification of the sector; PA1 (2) a 4 byte data word which is header data of the sector corresponding to the subcode Q data in the CD system; PA1 (3) 2048 byte data words which are user data; and PA1 (4) 288 byte data words which are used for detection and correction of the data error of (1) and (2) in mode 1 in the CD-ROM system, or for user data. PA1 (1) The address data corresponding to objective data stored on the disc transfers from the host computer to the CD-ROM disc reproducing apparatus. PA1 (2) The CD-ROM disc reproducing apparatus moves the optical pickup device to the address of the objective data stored on the disc. PA1 (3) After moving the optical pickup device to the address of the objective data, the objective data is read out and written in the buffer memory in the CD-ROM disc reproducing apparatus. PA1 (4) After writing the objective data, the CD-ROM disc reproducing apparatus transfers the objective data to the host computer at a constant velocity. PA1 (a) The position of the optical pickup device is nearby the position of the data required when the host computer transfers the command for accessing the data. PA1 (b) The position of the optical pickup device is far from the position of the data required when the host computer transfers the command for accessing the data. PA1 rotating means for rotating the disc; and PA1 data processing means having a plurality of optical pickup devices for reading the data from the rotating disc, wherein each pickup device reads a different part of the data area.
The storage format mode 1 is used for restoring or reproducing sound data. The data error correction ratio in mode 1 is 10.sup.-12. The data error correction ratio in mode 2 is 10.sup.-9 but the 288 byte data words are able to be used as user data. Accordingly the total user data in the mode 2 is 2336(2048+288) byte data words.
The 1 sector corresponds to a frame of data of subcode. The reproducing apparatus reproduces the data of the sector at 1/75 sec. The data transference rate is 2K.times.75=150 Kbytes/sec in the mode 1,so one sector is 2 Kbytes(2048 bytes). Accordingly the total data stored on a side of the disc which stores one hour of the audio data is 150.times.75.times.60=540 Mbytes. The CD-ROM disc is the recording medium which has a large data capacity and a low reproduction error ratio. The disc is able to store the computer data with the audio data. The computer data is distingished from the audio data by inspecting the subcode Q data.
The CD-ROM disc is only a recording medium, the CD-ROM system needs the host (computer) system. The CD-ROM system is controlled by the host (computer) system. When the host (computer) system transfers a command for accessing the data to the CD-ROM system, the CD-ROM system searches for the data accessed by the host (computer) system. After searching for the data, the CD-ROM system transfers the data located to the host (computer) system and the host (computer) system processes the data translated.
The data transference speed of the host computer depends on performance of the CPU (Central Processing Unit) of the host computer or application software program stored in the host computer. Therfore, the data transference speed of the host computer is different from the data transference speed (150 kbyte/sec) of the CD-ROM disc reproducing apparatus. The data transference speed of the host computer is not syncronized with the data transference speed of the CD-ROM disc reproducing apparatus. The CD-ROM disc reproducing apparatus has a buffer memory for absorbing the difference of speed between the data transference speed of the host computer and the data transference speed of the CD-ROM disc reproducing apparatus.
The process of the data transference from the CD-ROM disc reproducing apparatus to the host computer is represented as follows.
The CD-ROM disc reproducing apparatus repeats the process of steps (2) and (3), and completes the transference of all the objective data.
The CD-ROM system has a large data capacity and the storing density of the data is high. However, the time for reading out the data is long in spite of the CD-ROM system being a random data accessing system, so the disc revolution speed changes at the range of 500.about.200 rpm. The cause of the long read out time is changing of the disc revolution and the spiral data recording form on the disc.
For example, the read out time is 1 hour(3600 sec) when the CD-ROM disc reproducing apparatus reads out all of the data stored on the disc at 150 Kbyte/sec wherein the disc has 12 centimeter diameter and stores 540 MByte data word. In the case of reproducing audio data, it is no problem, but in the case of displaying the picture obtained by reproducing the data on the disk, laying device of the host computer and requiring display of a large amount of data, it is inconvenient that the CD-ROM disc reproducing apparatus reads out large numbers of the data required at every access requirement from the host computer.
There are two data accessing methods.
In the case of (a), the disc reproducing apparatus is able to read the data required by kicking the lens of the pickup device in the tracking direction.
In the case of (b), for reading out the data required, the disc reproducing apparatus must move the pickup device at first and then move the lens of the pickup device to the position of the data required. In the case when the position of the optical pickup device is far from the position of the data required, the read out time of the data becomes long, because the disc reproducing apparatus must perform the two steps.
The lens of the pickup device is designed to be able to move plus/minus 400 .mu.m in the radius directions of the disc. If the distance between each track of the data stored on the disc is 1.6 .mu.m, it is possible to read out plus/minus 250 tracks of the data stored on the disc by moving the lens without moving the pickup device. The program area of the disc has 20625 tracks of data.
The case wherein present position of the pickup device is within plus/minus 250 tracks from the position of the data required corresponds to the case of (a). The case wherein present position of the pickup device is outside of plus/minus 250 tracks from the position of the data required corresponds to the case of (b). In this case, it is nessesary to move the pickup device and the lens to access the data.
The CD-ROM disc stores the data in constant linear velocity form for high storing density of the data. Reading out the data stored in constant linear velocity form needs complex controlling of the disc motor driving. The controlling block for driving the disc mortor is mainly composed of AFC (Auto Frequency Control) block and APC (Auto phase Control) block. A frequency control signal is generated by comparing the frequency of the frame syncronizing signal and the frequency of the clock pulse(2.1168 MHz) for detecting the frequency of the signal in the AFC block. A phase control signal is generated by comparing the phase of the frame syncronizing signal divided and the phase of a standard frequency signal in the APC block. CLV (Constant Linear Velocity) control is operated by the frequency control signal and the phase control signal.
A conventional disc reproducing apparatus which reproduces the data stored on the CD-ROM disc requires a long time to read out all of the data stored on the disc.